Michelle's dress turns into fashion gold

This image of the US President and first lady embracing has become the most retweeted photo in history.

Barack Obama tweeted ''Four more years'' along with their photo embrace after winning the election and the photo was retweeted 815,824 times and made a favourite 297,755 times by Twitter users.

But, as Mr Obama was making history, so was ASOS, Britain's largest online-only fashion and beauty store and the stockist of the $US86 ($83) dress that Michelle Obama is wearing in the photo.

The dress sold out the first time she wore it; the retailer has not been able to meet demand since then.

ASOS has 2.1 million likes on its Facebook page, 411,000 Twitter followers and 315,000 followers on Instagram - and whenever a dress such as Mrs Obama's chic red and white checkered print dress is put on social media it is an instant sell-out.

An ASOS marketing manager, Prue Thomas, said social media had become a powerful tool and major commercial opportunity for the fashion industry.

''As soon as [as a celebrity] tweets a picture of themselves wearing a dress, there is a huge spike in sales and it instantly sells out. Everyone wants their hands on that item,'' Ms Thomas said.

''That one shot of Michelle is fashion gold for us.''

Facebook is the No.1 social media sales driver, she said, but the photo-sharing tool Instagram is on the rise.

Camilla Freeman-Topper of Camilla and Marc said social media was an ''incredible direct link'' to their customers.

''Editorial in magazines takes a while for the effect to trickle through [to customers] while having social media has immediate, direct effect,'' Ms Freeman-Topper said.